The present invention relates to electrohydraulic actuators adapted to operate cylinder valves. More particularly, the present invention relates to an electrohydraulic actuator assembly that couples a high speed solenoid switch with a stem valve, such as a poppet or cylinder valve, to control the flow of hydraulic fluid to a cylinder valve or piston mechanism.
It is well appreciated in the art that significant benefit can be achieved through variation of the lifting and timing of intake and exhaust valves in an internal combustion engine. For example, engine performance can be enhanced by individually controlling the acceleration, velocity and travel time of the valves.
Many hydraulic systems for controlling cylinder valves have been proposed. For example, U.S. Pat. No. 5,255,641 to Schecter for a VARIABLE ENGINE VALVE CONTROL SYSTEM describes a system in which the engine valve has a piston attached to its top. Opposite surfaces of the piston are subjected to hydraulic fluid. Selective activation and deactivation of a controlling means, such as a solenoid actuator, causes the hydraulic fluid to act on the appropriate surface of the piston, which causes the valve to move.
One problem encountered in the acceptance of hydraulic control systems is the need for a very fast response time. For example, in high speed engines, the response time for engine valve activation in a full stroke typically has to be within 2 milliseconds (ms). Controllers in electrohydraulic systems have thus far not provided the desired response times. For example, solenoid actuators, which are frequently used in electrohydraulic systems, have limitations on their response time, due, at least in part to the presence of eddy currents in the metal armatage element of the valve. The term xe2x80x98eddy currentsxe2x80x99 refers to the electrical currents that oppose the penetration of flux in the metal which is responsible for developing electromagnetic forces in solenoids. Eddy currents provide a natural inefficiency to electromagnetic systems, such as solenoid valves, because they limit the speed at which a magnetic field can be switched.
Powder metals provide materials that are mainly metallic, but have low conductivity which greatly reduces eddy currents. Typically, a powder metal is a compacted form comprising metal particles, such as iron particles, encased in a non-metallic material, such as an epoxy resin. These materials essentially prevent eddy currents because of the low conductivity due to the metal particles not being in contact with each other. As a result, powder metals can be used in electromagnetic systems to overcome the natural inefficiency due to eddy currents.
The present invention provides an actuator assembly that utilizes a high speed solenoid in conjunction with a stem valve, such as a poppet valve, to control the flow of hydraulic fluid to an engine intake or exhaust valve. The solenoid has an armature element formed of a powder metal, which essentially prevents eddy currents and allows the solenoid to generate the electromagnetic force to provide the desired response time. As a result, the present invention provides an electohydraulic actuator that allows for effective variable valve lift and timing control.
In one embodiment, an actuator assembly according to the present invention comprises a cylinder valve, such as a poppet valve, that is moveable between open and closed positions, a bias spring biased to keep the valve in either the open or closed position, and a solenoid actuator having an armature element formed of a powder metal and adapted to control the movement of the valve between the open and closed positions.
The present invention also provides a control assembly for operating a two stage cylinder valve. The control assembly incorporates two actuator assemblies of the present invention, one of which is connected to a high pressure source of fluid and the other of which is connected to a low pressure source of fluid. The solenoid actuators of the actuator assemblies control the position of the respective valves, which ultimately control the type of fluid that flows to the cylinder valve.